Lee, U.S. Pat. No. 4,792,367, issued Dec. 20, 1988, demonstrated that very fine grained compositions of certain transition metals including iron, rare earth elements including neodymium and/or praseodymium, and relatively small amounts of boron can be suitably hot worked to form very strong anisotropic permanent magnets. Lee's process is applicable to compositions of the type disclosed by Croat in U.S. Pat. No. 4,802,931 issued Feb. 7, 1989.
Croat disclosed permanent magnet compositions containing as the essential magnetic phase very small grains of tetragonal crystals of RE.sub.2 TM.sub.14 B where RE is one or more rare earth elements including neodymium and/or praseodymium, and TM is preferably iron or mixtures of iron and cobalt. While RE.sub.2 TM.sub.14 B (for example, Nd.sub.2 Fe.sub.14 B) is the essential and predominant phase, preferred compositions also contain a relatively small amount of one or more grain boundary phases containing rare earth elements and transition elements and sometimes boron. Typically, the grain boundary phase is richer in rare earth element content than the principal phase. The grain boundary phase which surrounds the larger grains of the RE.sub.2 TM.sub.14 B phase is believed to provide magnetic coercivity in such material by pinning the magnetic domain walls formed in the larger grains when the material is placed in a magnetic field. In general, suitable overall compositions for the preparation of such permanent magnets comprise in terms of atomic percentage about 50 to 90 percent transition metal, about 10 to 40 percent rare earth metal and at least 0.5 percent boron.
Alloys of such composition were melted and very rapidly solidified such as, for example, by melt spinning to produce a fine grain microstructure. The material was processed to obtain a material in which the average grain size of the principal phase was in the range of 20 to 300 nanometers. Materials of such microstructure could be obtained either directly upon melt spinning or by a practice of overquenching to an amorphous material and annealing to obtain the desired grain size. These practices are disclosed in the above-identified Croat patent.
The Croat-type compositions had appreciable coercivity and in general were magnetically isotropic. The melt-spun or melt-spun and annealed particles could be pulverized if desired into a powder of average size of a few microns to 350 microns. The powder could be consolidated with a suitable resin to form a unitary magnet body having no preferred direction of magnetization. Such magnet materials have many useful applications. However, their maximum magnetic properties are not appropriate for applications in which higher strength anisotropic magnets would better serve.
Lee's patent describes the hot pressing of the Croat magnetically isotropic powder to form a full density magnetic body that was generally isotropic but displayed some magnetic anisotropy in the direction in which the particles were pressed. However, Lee found that upon further hot working of his original hot pressed compact, even stronger, more definitely anisotropic permanent magnets could be formed. Further development of the Lee practice has centered on the hot working techniques for the iron-neodymium-boron type materials so as to achieve ever more complete alignment of the 2-14-1 grains and greater anisotropy and magnetic properties. The term "2-14-1" is a shorthand reference to RE.sub.2 TM.sub.14 B grains or to compositions containing or based upon such a tetragonal crystalline phase.
Sometimes in the manufacture or usage of these strong permanent magnets it is desired to demagnetize them to facilitate handling or shaping. It is known that these magnets can be demagnetized by heating them above the Curie temperature for a few minutes. The Curie temperature of these 2-14-1 magnets depends upon their composition but is typically of the order of 300.degree. C. to 500.degree. C. The difficulty with demagnetizing these magnets by this practice is that the magnet then possesses a lower value of coercivity than it has in its manufactured state. It is an object of the present invention to provide a method of demagnetizing hot worked, fine grain 2-14-1 type magnets such that any loss of coercivity is minimized or eliminated. In many instances, it is possible to obtain a modest increase in coercivity by the practice of this invention.